Diff Checker
Compare two blocks of text and instantly see additions, deletions, and modifications with character-level highlighting. Perfect for code reviews, document versioning, and finding changes between files.
How It Works
- Paste your original text on the left
- Paste the modified version on the right
- Click Compare to see all differences
- Toggle between unified and split views
- Green highlights show additions, red shows deletions
Features
- Line-by-line comparison with LCS algorithm
- Character-level highlighting within modified lines
- Unified view (interleaved) or split view (side-by-side)
- Real-time line count display
- Swap inputs with one click
- Works entirely in your browser, no data leaves your device
What is Diff Checker?
Diff Checker is a free online text comparison tool that instantly highlights the differences between two blocks of text. It uses the Longest Common Subsequence (LCS) algorithm to detect additions, deletions, and modifications at both the line level and the character level, giving you precise visibility into exactly what changed. No sign-up required, no file uploads needed, and your data never leaves your browser.
Whether you are reviewing code changes, comparing document revisions, or verifying configuration edits, this diff checker gives you a clear, color-coded breakdown of every difference. It runs entirely client-side, so even sensitive or proprietary text stays private on your device.
How to Use This Tool
Follow these steps to compare text and find changes with this free online diff checker:
- Paste the original text - Place your original version into the left text area. The tool shows a live line count as you type or paste.
- Paste the modified text - Place your updated version into the right text area. You can use the Swap button if you accidentally put them in the wrong order.
- Click Compare - The tool runs the LCS diff algorithm and generates a color-coded output showing every addition (green), deletion (red), and modification (yellow) with character-level precision.
- Switch views - Toggle between Unified view (interleaved changes, similar to git diff) and Split view (side-by-side comparison) using the view selector in the toolbar.
Key Features
- Character-level highlighting - Within modified lines, the tool highlights the exact characters that were added or removed, not just the entire line. This makes it easy to spot small typos or single-word changes.
- Unified and split views - Switch between an interleaved unified diff (old and new lines together) or a side-by-side split view for easier visual comparison of long documents.
- Change statistics - A summary bar shows totals for added, removed, modified, and unchanged lines, giving you a quick overview of how much changed.
- One-click swap - Accidentally pasted text in the wrong panel? The swap button instantly reverses the original and modified inputs without losing content.
- Complete privacy - All processing happens in your browser using JavaScript. No data is sent to any server, making it safe for comparing confidential code, legal documents, or private notes.
Common Use Cases
Developers use this diff checker to review code changes before committing, comparing an old function with a refactored version to verify nothing was accidentally removed. Technical writers compare document drafts to see exactly which sentences were reworded or paragraphs added. System administrators paste configuration files side-by-side to audit what changed after an update or deployment. Students compare essay revisions to verify that feedback was addressed without introducing new errors.
Frequently Asked Questions
Is my text data safe when I use this diff checker online?
Yes. This tool runs entirely in your web browser using client-side JavaScript. Your text never leaves your device and is not sent to any external server. You can even use it offline after the page has loaded, which makes it safe for comparing proprietary source code, legal contracts, or any sensitive content.
What is the difference between unified view and split view?
Unified view shows all changes in a single column with additions and deletions interleaved, similar to how git diff displays changes. Split view shows the original text on the left and the modified text on the right, aligned line by line. Unified is better for reading through sequential changes; split is better for visually comparing two versions side by side.
How does the character-level highlighting work?
When two lines are detected as modified (rather than purely added or deleted), the tool runs a second LCS pass on the individual characters of those two lines. This identifies the exact character spans that differ, highlighting them with background color so you can immediately see a single changed word or character within an otherwise similar line.
Is there a maximum text size or line limit?
There is no hard-coded limit on the number of lines you can compare. However, since the LCS algorithm uses a matrix proportional to the product of both inputs' line counts, very large files (over 10,000 lines) may take a few seconds to process depending on your device. For character-level diffs, lines longer than 500 characters skip the character comparison for performance reasons.
Can I use this diff checker for comparing code files?
Absolutely. The tool displays content in a monospace font with line numbers, making it well-suited for code comparison. It preserves whitespace and indentation, and the character-level highlighting catches small changes like renamed variables or modified operators. It works with any programming language or markup since it compares plain text without language-specific parsing.